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Replacing or repairing TR-808 switches and pots

2014-02-12  Robin Whittle rw@firstpr.com.au


"Death and taxes are unsolved engineering problems" - Romana Machado.

Introduction

This page discusses the repair or replacement of all the TR-808's switches and pots, with a final detailed section on the Run/Stop  / Tap keyswitch.  There was an unsolved engineering problem regarding these switches.  They fail and are not easily replaced or repaired.  However, there is a promising replacement kit from Social Entropy: http://socialentropy.com .

You may also be interested in my analysis of tact switches for the TB-303:

../dfish/tact-switches/   ../dfish/303-mods/

The TR-808 Service Manual can be obtained from:

https://archive.org/details/synthmanual-roland-tr-808-service-notes

The TR-808 is a mains-operated device with exposed mains connections on the power switch and the power supply board.  Working on this machine while it is plugged into a mains socket constitutes a serious safety hazard.  To so so risks serious injury and DEATH.  If you are not an experienced electronic technician who already knows about mains safety and about static electricity damage to semiconductor devices, I suggest you not dismantle this machine at all.


TR-808 pots

#Volume-Pot
The Volume pot may become noisy.  My fix for this is to de-solder it, dismantle it by bending tabs, and then to clean the contacts with isopropyl alcohol. I usually spin it around a few times (be sure to do so in the direction where the wipers are dragged, rather than pushed) and use little strips of cardboard soaked in isopropyl alcohol.  As far as I know, this fixes the problem for a few years at least.

Technology Transplant http://www.technologytransplant.com/ sell newly manufactured Volume and Tempo pots.  I haven't seen any trouble with Tempo pots, but they too could become erratic and benefit from cleaning with isopropyl alcohol.  Likewise the Fine Tempo pot.

Generally, I advise never spraying anything anywhere inside electronic devices.  This is especially so for various fluids which contain oil.  However, if you are not in a position to de-solder, dismantle, re-assemble and re-solder a noisy pot, it might not be such a bad alternative to squirt some pure isopropyl alcohol into the pot, give it a few dozen turns, and leave it for an hour or so to dry out.  It is hard for me to imagine this leading to any difficulties, and there's a good chance this will fix any noise problems, at least for the coming months or years.

#Small-Pots
I have not seen any serious trouble with the many small pots of the TR-808.  However, they can become noisy.  Technology Transplant used to sell replacement sets, so it would be worth contacting them if you want a whole new set.  However I can't imagine why anyone would need such a set, unless they had been used far more than the pots in TR-808s I have worked on.

In August 2012 I had success systematically cleaning all these pots in the following manner.
  1. Squirt in a little isopropyl alcohol into all the pots.
  2. Turn each pot one or two dozen times.
  3. Squirt in a little more isopropyl alcohol.
  4. Use compressed air to blow out the isopropyl alcohol, while turning each pot.  This inevitably causes isopropyl alcohol to go elsewhere on the PCB, in addition to where it went after the first two squirts.
  5. Leave the whole machine to dry for an hour or so before turning it on again.

TR-808 switches

The section after this one concerns the Start/Stop and Tap switches.  This section concerns all the others.

I have never seen any problems with the three Rotary switches.  It appears that ALPS still make this range of switches: 

http://www.alps.com/WebObjects/catalog.woa/E/HTML/Switch/Rotary/SRRM/SRRM_list.html

however, it could be tricky to find a supplier with the exact same type for a one-off order.

There is a little push-button switch for Pattern Clear.  They are a horrible-feeling switch, with no clicking action at all.  I haven't seen one fail. They are probably easy to replace since switches such as these, with integral buttons, are widely available.

There is a 3 position Basic Variation toggle switch and a 2 position I/F Variation A B switch.  Technology Transplant sell a pair of these, newly manufactured:

http://www.technologytransplant.com/product_info.php?cPath=5_13&products_id=47

There is a 3 position slider switch for Pre Scale.  In my experience, this type of switch can become erratic due to dust, migrating grease and/or corrosion.  My first approach to dealing with this would be to squirt some isopropyl alcohol into the switch and move it back and forth a hundred times or so.  It is possible to dismantle them for cleaning, but it is tricky getting them back together, and I am not sure that this would be superior to leaving it in-situ and putting some isopropyl alcohol inside it.  Likewise the Sync Input / Output switch on the back panel.

I have never worked on the Power switch.  Some old Roland mains-powered guitar processing boxes had a similar switch.

#Tact-Switches
The 16 Tact Switches are a particular concern.  Much of what follows would also be relevant to the JP-8 (Jupiter 8) which uses the same switches.  If you have whole sets of these failing, such as groups of four, then you may have a problem with the printed ribbon cable connecting to the switch board.  This is easily fixed by replacing the ribbon with direct wiring from PCB to PCB.

The TR-808 tact switches are ALPS SKHCABs.  These are a stemless version of the SKHCAA type used in the TB-303 and TR-606.  These are still produced and are known as SKHCBFA010 for the stemless version (TR-808 and JP-8) and  SKHCBEA010 for the TB-303 type with stems.  These are both "1.27 newton" switches, which means about 124 grams as a force.

http://www.alps.com/WebObjects/catalog.woa/E/HTML/Tact/SnapIn/SKHC/SKHC_list.html

I do not recommend using these ALPS switches as replacements.  Nor do I recommend using the ALPS sealed tact switches, since they have what I consider to be a very poor action.  Other people find this acceptable or desirable in the TB-303, but I can't imagine how they could be better than the Omron switches discussed below.  See ../dfish/tact-switches/ for measurements and discussion.

I recommend using Omron B3W-4000 sealed tact switches to replace the ALPS ones used in the TR-808.  These have a similar feel and are sealed against liquids and dust.  So I expect them to last for many years.

The Omron page for these is:

http://www.components.omron.com/components/web/webfiles.nsf$FILES/family.html?ID=CNEN-6TJQQR

They are available at Farnell, element14 or whatever they call themselves today: http://www.element14.com . Mouser has them at a better price, but the illustration on their page (2012-03-12) shows another switch. http://www.mouser.com .

One approach was  to buy a complete replacement switch panel from Technology Transplant.  However, in early 2014, this product is no longer available.

http://www.technologytransplant.com/product_info.php?cPath=5_13&products_id=255

Technology Transplant replacement tact switches for the Roland TR-808 TR808 drum machine
16 pc. Sequencer Step Switch Assemblies + Caps PCB
"Assembled for quick install. Shave 1 hour of tech time and completely overhaul the feel and use of your thumper!"


These used newly manufactured actuators (black plastic frame)and buttons.  Roland got these black plastic actuators and coloured buttons from ALPS in the early 1980s.  John at Social Entropy found an obscure ALPS catalogue page which lists the black actuator, with red LED and tact switch, but without a coloured button, as KHC 11901.  Here is the image of the sample-card / catalogue page:

1980s-ALPS-switch-catalogue-sample-card.jpg

Assuming these are the same as what is used in the TR-808, JP-8 etc, the switch is a 130 grams force "KHC 10902", which must be the older name for what I knew as an SKHCAB, the stem-less version of the SKHCAA (as used in the TB-303) which is now (after ~2009?) known as an SKHCBEA010.  ALPS tact switches, such as the SKHCABs used in the TR-808, have four little slots in the underside of the body which seem to be for this style of clip-on actuator.

Searching for "KHC10902" turns up links to service manuals for the Korg Monopoly and PolySix.  I recall these use the same tact switch and actuator, but the buttons are a different shape and colour.  My hypothesis is that while ALPS supplied the tact switch, actuator and LED, probably as a single component ready to be inserted in the PCB, they did not supply the buttons.  Therefore, the Red, Orange, Yellow and White buttons used in the TR-808 (and the JP-8 has Blue) would have been Roland parts.

Technology Transplant had good replicas of the actuators and buttons made for their TR-808 switch board.  The new LEDs in their boards were  somewhat brighter than the originals, without being excessively bright.

However, the buttons were not absolutely identical in shape or colour, so it wouldn't look quite right to install one of these buttons in place of one which went missing on a TR-808.

A common problem is that a button is missing.  This is usually not just the button which has come adrift, but part of the black plastic actuator as well.  Therefore, the entire actuator needs to be replaced.  The actuator contains a LED and snaps around the switch.

In principle, someone could 3D print new buttons.  However, it would be difficult to get the right colour, and I expect there would need to be some manual work smoothing the printed surface to get a nice texture for the part which we touch with our fingers.  3D printing a black actuator would be easy in terms of colour, but really tricky in practice, since it uses a thin piece of plastic as a hinge.  This would be really difficult to reproduce in terms of flexibility and longevity, without the original injection molded polypropylene (I guess) plastic.

The Technology Transplant TR-808 switch board was a most welcome development, since it was the only source of replacement actuators and buttons.  However, they use the same kind of unsealed tact switches as Roland used - so these switches will fail within a few years, due to dust.

It is possible to significantly prolong the life of these unsealed tact switches by placing a thin strip of Mylar (technically biaxially-oriented polyester/PET) or other plastic between the tops of the switches and the bottom of the flexible part of the actuator.  I used Mylar film from 3M "Professional Flip-Frame" overhead Transparency Protectors (RS7110).  The strip is a little wider (left-right) than the whole set of buttons and, I recall, about 20mm long (front-back).

However, I believe it is better to use Omron sealed tact switches instead of the new switches in these Technology Transplant switch panels.

Here are some photos of the original ALPS tact switches, the actuators and the buttons.



The actuator has dual thin plastic hinges between the moving part which attaches to the button and the fixed part.  This can fatigue with excessive force, or perhaps just lots of use.  When this happens, this part of the actuator and the button will come loose and fall off the machine.  Please be careful if you remove the button, since the force could break the hinge.

There's no way of repairing this broken actuator, so (sadly, no longer available in 2014) the Technology Transplant replacement boards were highly desirable.  However, if someone installs a complete new board, this means they have an old board with at least some good actuators in them.  I have one such board and would be interested in acquiring one or two more, so I can use the original actuators and buttons to replace occasional broken actuators on machines I am working on.  If you have replaced your original board with a Technology Transport one, and would like to sell me the original board with most of its buttons and actuators, please let me know: rw@firstpr.com.au .




This photo shows that the actuator has four little hooks by which it is aligned with the top surface of the PCB by clipping under the ALPS tact switch, on the left.  There are four slots molded into the switch body for this purpose.  In August 2012, the machine I worked on had no hooks at the rear of the actuator body.  The hooks in the front (lower on the above photo) are minimal.  The above photo gives the appearance of hooks at the back, but I am not convinced that these really exist as functional hooks - I suspect they are molding marks.  Perhaps the design was changed.  There's no strong reason to have hooks at the back, since the actuator is held in place by the LED wires being soldered to the PCB.

The Omron switch on the right has no such slots.  Therefore, we need to file away at these four bottom corners of the Omron switch body.  In August 2012 I decided that we really only need two slots, on the end of the switch which will be at the front (lower on the following photo).



The Omron switch bodies are about 0.1mm higher off the PCB (downwards in the above photo) than the ALPS switches.  Since we rely on these clips at the front of the switch (bottom of the above photo) to hold the actuator frame flush with the PCB, it is quite tricky filing the right amount of material off the switches.  Too little and the clip does not go in at all.  Too much and the clip will not hold against the switch body as intended.  For doing this faster I used a small milling bit from a Dremel tool, affixed to a drill press, with a guide below.  This is tricky, so for a single set of switches I suggest you just use a file.  It may help to cut down or file down the two locating lugs in the middle of the switch, so you can get a more shallow angle cut when filing.

Installing Omron sealed tact switches is quite a laborious and tricky task.  It is worth it, I believe, since these switches feel good and will probably last for decades.

#LEDs

It is possible to install new LEDs while doing the above operation.  Modern, more efficient, LEDs may be excessively bright.  This can be fixed by increasing the value of R60, R62, R64 and R66 on the main board.

Also, it is very tricky to bend the LEDs to the exactly correct dimensions and insert it into the actuator.  The existing LEDs have fine leads and are held in place by heat-deformed plastic.  It is a lot of work to remove the LEDs, prepare the actuators for new LEDs, bend the new LEDs' leads correctly and insert them in the actuators, so when soldered they are all at the correct height.  I think few modern LEDs have fine enough leads to make this a straightforward task.  Generally this would be done by removing the buttons to give access to the LED with its pre-bend leads to the holes for these leads.  However, it is desirable to do this work without removing the buttons, so as not to place any extra stress on the plastic hinges of the actuator.  I did figure out a way of doing this, but it is awkward.  The new LED can be put through the hole of the button, with its leads going straight down through a slot there, not in the path of the old LEDs leads.  Then they leads can be bent sideways to meet the PCB (this may involve using a Dremel etc. tool to remove some plastic from the base of the actuator).  However . . . the new LEDs need to be narrow enough to fit, and they are probably not the right height. So the new LEDs must have one or two small pieces of tubing put around the leads, to lift the base of the LED body from the top of the slot in the actuator, while allowing the downwards force on the leads and so the LED body to press the actuator down to the PCB.  The LED leads and body play a crucial role in holding the rear end of the actuator to the PCB.

A major advantage of the Technology Transplant panels is that they come with new LEDs which are moderately brighter than the old ones.

My inclination is not to replace these LEDs.  It is a lot of work and I think the old LEDs work OK.

Installing LEDs in multiple colours would be tricky, especially due to the fact that they are driven in groups (see the schematic for details) which means it would be difficult to adjust for different LED efficiencies except by installing individual resistors in series with each LED.



Start / Stop and Tap switches

#SSTap
The above information provides what I consider to be reasonable ways of coping with failure of most of the TR-808's pots and switches which are likely to fail.

In 2014 there is an important new development:  Social Entropy are developing a replacement kit for the two switches and their white plastic buttons.  This kit is forthcoming - it not yet available for sale.

Please contact them: http://www.socialentropy.com/quicksilver/contact.html about these kits. It consists of a modern keyboard switch, soldered to a specially designed PCB with pins, which brings it to the correct height.  Since the + pattern of the new switch, where it connects to the button, does not suit the TR-808's white plastic buttons, there is a very well crafted, 3D printed, white plastic button in place on the switch. (This can be removed, but there is no need to do so.)  The idea is that the label and the outer transparent shell from the TR-808 are affixed to the new button.



In February 2014 I installed these in a customer's machine.  (Someone had sprayed oil everywhere and this had made the original switches completely non-functional.  The outcome was a total success.
This approach is clearly superior to any other solution I know of.

The following is what I wrote before this kit became available:


The first subsection here concerns the original switches and how they might be refurbished.  In a further subsection below that I consider alternative switches.

All the switches I have had to replace have been of the type shown immediately below, with a white stem and two melted lugs to hold the switch together.  One problem refurbishing these is how to get the switch back together.

Another type has a black shaft and side clips to hold the switch together.  If one of these is in a TR-808 (I have not seen one so) then refurbishing these is a lot easier because the switches can be snapped apart and back together with ease.  If you are fixing a switch of this latter type, please read the next section on cleaning the contacts, but it will be obvious how to put your switch back together.

Refurbishing the existing switches - melted lug sealing

#Tap-refurb
Here are some photo of original switches.  It is made by ALPS but I don't know its part number.  I don't know of any replacement buttons either, but generally these are not damaged, destroyed or lost.





It is possible that later TR-808s had a different type of ALPS switch with the same dimensions, but a different form of construction, as described below.  For simplicity I will refer to the above type as the "original switches".

The key point about them is that the top of the body is affixed to the bottom part, the bottom 3.5mm, by two heat-melted cylindrical lugs.  We need to "drill these out" to dismantle the switch, though in the past I recall I cut the switch apart with a knife, which was a bad idea.

I used a small Dremel tool to cut away the ends of the melted lugs.  It doesn't take much, just a fraction of a millimetre, before the switch can be prised apart with a knife-blade in the split.  By cutting a circular path following where the lug has been melted sideways a little, it is possible to leave the centre of the lug intact, but still cut away enough of its widened end to make it possible to prise the switch apart.



The failure mechanism is the lack of conductivity when the conductive rubber presses against the metal contacts.  One of these switches was 127k ohms when pressed.  It needs to be about 10k or less to activate the circuit.

 My usual approach to dealing with rubber contacts is:
  1. Use some light, clean, fresh, emery paper (dark-grey paper-backed abrasive sheet) to abrade away the surface of the conductive rubber.  (I don't put any solvents on this, because once when I did, the rubber became less conductive.)  240 or 400 grit is probably a good choice.

  2. Use isopropyl alcohol to clean the metal contacts.

  3. Maybe use a very light touch of fine, fresh, emery paper to roughen the surface of the contact points just a little.  800 grit is probably a good choice.
In this case, point 3 is a must.  The contacts are selectively gold plated and examination with a stereo zoom microscope revealed a dark shiny patina on the gold.  In both these switches, one contact was darker than the other, so perhaps there was some kind of electrical process affecting the deposition of material.  In the one switch on which I tried acetone, before any abrasion, the acetone appeared to take off the lighter coating on one contact while not affecting the darker coating on the other

Scratching with a needle cuts through this to the gold and copper below.  The 240 grit emery paper scratched up the surface revealing (under the microscope at 30x) some copper rather than gold. This is not ideal, since the copper is subject to corrosion.  Still, it is better than retaining whatever coating had built up on the gold surface.  When I used 800 grit emery paper, very lightly, under the stereo zoom microscope, I was able to get a gold-looking set of scratches, without obvious copper colour.

An additional step which is possible with these switches is to rotate the rubber cup so that fresh material is exposed to the raised contacts.  However, once the conductive rubber is sanded, it is not obvious where it was pressing into the contacts.

Using these techniques I was able to make one of these switches go consistently below 1k ohms - 800 to 900 ohms.  However, the other was 2k to 3k and went higher, beyond 10k, as I attempted improvements, such as smoothing down the abraded metal contacts.  I assume the rubber contact material has degenerated in some way, perhaps due to some solvent which might have been on contact with it.  However, perhaps my abrasion was too rough.

There are some special paints which are intended to fix problems with these conductive rubber switches.  I haven't used any and I don't at present know which ones are well-regarded, but such a product might save the second switch.  A quick search found this product: http://www.keypadrepair.com .

Also, it would be possible to modify the circuit so it was acceptable to have a 10k or higher resistance.

Assuming the switch contacts can be made to work acceptably, this still leaves a problem with how to fix the switch back together.  One approach is to use a soldering iron to melt the lugs again.  This would probably work, but next time the switch needs to be refurbished, it will be difficult to do this again.

In the past I tried epoxying these switches together, but that was after a careless disassembly which I think involved cutting around the seam of the switch.  I buttressed the sides with epoxy with a filler, going down to the PCB, but still the switches fell apart after a little use.

Here is a method I devised in March 2012 which I think will work well.






I would solder the twisted joints and probably give them a bit more of a twist to tighten them up further.

This looks a bit rough, but I think it will be strong enough for long-term use.  It is easy to pull it apart in the future.  Although I filed slots in the bottom, the wire is below the base of the switch so some care or insulation tape or the like will be required when mounting the switch in the TR-808.


Replacement switch from an MC-4

#Tap-side-clip

I was lucky enough to obtain a few of these switches, which have never been used used, as spare-parts for the keyswitch panel of an MC-4 micro-composer.  These are physically compatible with the switches above, and perhaps they are used on some TR-808s.

They operate on the same principles, so refurbishing the contacts would be along the lines described above. 
This one had a resistance of about 500 ohms.





Both types of switch are intended to snap into a punched steel plate for the purposes of making a numeric, QUERTY or other similar kind of "keyboard" set of switches.

The sides unclip easily to reveal:







I believe these are a later model, superior, switch to the melted lug version.  The melted lug version used deformation of the silicone rubber cup to take up the slack of the button moving down past the point where the contacts closed.  This one uses a coil spring, and I believe this places less stress on the silicone rubber cup.

Does anyone know a source of these switches or anything like them?


Purchasing alternatives?

#Tap-alt

Other than the above mentioned Social Entropy kit, which seems to be an excellent solution, I don't know of any alternative switches.  In a July 2011 discussion:

http://music-electronics-forum.com/t23676/

this switch http://www.smallbearelec.com/Detail.bok?no=36 was suggested, but it only has two pins and is apparently a different height.

It can be safely assumed that the people at Technology Transplant have a better idea of what is manufactured, and can be manufactured, than most folks.  Yet they have no such switches.  I think this is strong evidence that no such switches are currently being manufactured.

Does anyone have suggestions?

These switches measure 13.5 x 13.5mm in the main body - this is the size of hole they are intended to be mounted in.

The four pins are spaced identically to the ALPS and Omron switches.  I was tempted to think of some adaptation of the existing body and slider mechanism with an Omron tact switch replacing the current base of the switch.  Another idea is to keep the existing switch body and base, but have a hole through the bottom of it and the PCB, with a shaft driving an Omron switch mounted below the PCB.



Update history:


© Robin Whittle 2012 2014 First Principles and Real World Interfaces
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